1. Technical Field
The present invention relates to a surface acoustic wave device using a C-plane sapphire substrate, an oscillator, and a module apparatus.
2. Related Art
A surface acoustic wave device which uses surface acoustic waves propagating on the surface of a piezoelectric film is used in a band-pass filter for a communication instrument, a resonator serving as a reference clock, or the like because of a unique transmission band, reduction in size, and a small number of components. When a surface acoustic wave device is used in a filter or a resonator, it is necessary that a frequency temperature coefficient (TCF) or a delay time temperature coefficient (TCD) representing temperature dependency or an electromechanical coupling coefficient (K2) representing performance of electromechanical conversion is satisfactory.
A surface acoustic wave device has been suggested in which comb-like electrodes (IDT) are formed on an insulating substrate, such as glass, a piezoelectric film is formed to cover the surfaces of the comb-like electrodes, and a protective film is formed to cover the piezoelectric film (for example, see JP-A-10-178330).
A surface acoustic wave device has also been suggested in which a comb-like electrode is formed on a C-plane sapphire substrate and a piezoelectric film is formed to cover the surface of the comb-like electrode (for example, see JP-A-10-135773 and JP-A-8-130435).
In the surface acoustic wave device described in JP-A-10-178330, the insulating substrate, such as glass, is used as a substrate, and the comb-like electrodes, the piezoelectric film, and the protective film are formed on the insulating substrate in the form of a laminate, preventing penetration of moisture or foreign substances from entering and preventing deterioration or degeneration of the piezoelectric film due to an external atmosphere. However, if the material or the film thickness of the protective film is not sufficiently taken into consideration, the acoustic speed of surface acoustic waves, the electromechanical coupling coefficient, and temperature characteristics are changed, making it very difficult to obtain satisfactory characteristics.
In the surface acoustic wave devices described in JP-A-10-135773 and JP-A-8-130435, the comb-like electrode made of an aluminum-based alloy is formed on the sapphire substrate, and a zinc oxide (ZnO) film is formed to cover the comb-like electrode, improving stress migration resistance. In such a configuration, if the film thickness of the zinc oxide film is not sufficiently taken into consideration, the acoustic speed of surface acoustic waves, the electromechanical coupling coefficient, and temperature characteristics are changed, making it very difficult to obtain satisfactory characteristics.
When a zinc oxide is used for the piezoelectric film on the C-plane sapphire substrate, there is a great difference between the acoustic speed of the sapphire substrate and the acoustic speed of the zinc oxide, and the difference in the acoustic speed causes frequency fluctuation.